It is known that there is a trade-off relationship between the amount of NOx emission from an engine and the fuel consumption rate. It is desirable to operate the engine so as to reduce the fuel consumption rate while satisfying a NOx regulation value.
To restrict production of NOx, it is effective to control the ignition timing and the air-fuel ratio. For instance, Patent Document 1 (JP2003-148781A) discloses restricting knocking and production of NOx upon a change in fuel composition for a gas engine supplied with fuel with variable composition by controlling the ignition timing and the fuel-air ratio of the gas engine in accordance with the output of a fuel-composition measurement unit, a load measurement unit, and an engine speed measurement unit.